The "attitude" of a satellite describes the satellite's orientation with respect to a reference coordinate system (e.g., earth-based or star-based). The attitude of a satellite is fully described using three parameters (e.g., by pitch, yaw, and roll angles).
In a satellite communications system, precise knowledge of a satellite's attitude is essential to achieve efficient, low-power communications with ground sites or other satellites. Communications systems on-board the satellite need accurate attitude information to be able to direct their transmissions (e.g., radio frequency or laser), to a remotely located receiver. Accurate attitude information is particularly essential for communications at higher frequencies, which often rely on very narrow communications beams (e.g., hundreds of microradians).
Prior-art attitude determination systems typically use various combinations of rotation sensors (gyroscopes), star-referencing devices (sensors, trackers or mappers), earth horizon sensors, sun sensors, and/or magnetometers to determine a satellite's attitude. Each of these prior-art devices are described below, along with their various advantages and disadvantages for satellite communications applications.
Rotation sensors are prior-art devices that monitor a satellite's change in attitude. Because rotation sensors only monitor attitude changes, they alone cannot determine a satellite's absolute attitude. For this reason, rotation sensors are useful to maintain an attitude once it has been established, but not to determine an attitude initially or on an ongoing basis.
Prior-art star-referencing devices are useful when high-precision attitude determination is desired. High-precision attitude determination may be achieved because star-referencing devices use stars, which are unambiguous point sources of light, as reference points. A star-referencing device looks at a reference star (i.e., a star whose exact position is known to the satellite attitude-determination system) and may partially determine a satellite's attitude relative to that star. The relative attitude measurement can then be related to any reference coordinate system in which the star is located.
A complete description of the satellite's attitude may not be determined using a single star-reference device's measurements because one axis exists about which the satellite's attitude is not known. Specifically, there is an ambiguity about the line-of-sight from the satellite to the star. An additional measurement about another axis may be used to resolve the ambiguity. This single-axis ambiguity is inherent also in rotation sensors, horizon sensors, sun sensors, and magnetometers.
A star-referencing device has several drawbacks. First, the star-referencing device cannot provide attitude determination measurements when no reference stars are in view. When continuous attitude determination capability is needed, an attitude-determination system based on a star-referencing device would require an additional attitude-determination device (e.g., a rotation sensor) in addition to the star-referencing device to keep track of the attitude during the times when reference stars are not in view.
Another drawback is that a star-referencing device cannot operate when it is looking at reference stars within a few degrees of the surfaces of the Earth, Sun, or Moon, because their brightness could damage the device.
Additionally, attitude determination using a star-referencing device requires knowledge of the locations, brightness and sometimes colors of many reference stars. Thus, relatively large computer memory is required for star catalog storage and high processing capability is required for star-identification calculations.
Another drawback is that star-referencing devices are expensive, heavy, and require more power than other types of sensors.
Another prior-art attitude determination device is an earth horizon sensor which uses the "limb" (i.e., the boundary between space and earth) to determine a satellite's attitude. Like all the other prior-art sensors, the horizon sensor can not provide attitude information about one axis. A horizon sensor may determine all three angles of an attitude by waiting through a significant portion of a quarter of an orbit, taking new measurements, and combining the new measurements with previous measurements. Inaccuracies in the attitude determination are inherent in this method, however, because the previous measurements may not be completely accurate a quarter of an orbit later.
Earth horizon sensors are inaccurate due to the atmospheric "fuzziness" of the earth's horizon. Additionally, a horizon sensor must have a stored geoid model due to the non-spherical nature of the earth (e.g., equatorial bulge). Such a geoid model may add significant computational complexity. Another drawback is that a horizon sensor is calibrated to a particular altitude. Therefore, the satellite must orbit at that altitude to achieve accurate results.
A sun sensor is a prior-art device that determines attitude by establishing a line-of-sight between a satellite and the sun. Similar to other sensors, the sun sensor cannot determine attitude about the line-of-sight from the satellite to the sun. Sun sensors are relatively inexpensive, and require little power.
However, sun sensors can be used to determine attitude only when the sun's image is in view. Thus, a sun sensor cannot determine attitude when the satellite is in the shadowed portion of the satellite's orbit (in low-earth orbits, this may be up to one third of the orbit) or when the satellite's attitude is such that the sun does not lie within the sun sensor's field of view. Like star-referencing devices, this limitation necessitates an additional attitude determination device to determine the attitude when the sun is not within the sensor's field of view. Sun sensors may be less accurate than star-referencing devices because the sun sensor utilizes the sun's image as a reference point. Compared with a star which approximates a point of light, the sun is a large disk. Because the sun's center may be difficult to determine and locate, pointing inaccuracies are inherent in a sun sensor.
A magnetometer is a prior-art device that determines a satellite's attitude by measuring the earth's magnetic field at the satellite and comparing it with a magnetic field model at the same location. The magnetometer, like other sensors, provides no information about the measurement axis.
One disadvantage is that a magnetometer is not highly accurate. One reason is that a satellite's own magnetic field is also measured by the magnetometer, thus corrupting the magnetometer readings. Additionally, the magnetic field model may not accurately account for variations in the earth's magnetic field. This is especially true at higher latitudes (i.e., near the poles).
Two primary performance limiting factors in every satellite are the mass (or weight) and available electrical power. Satellite mass is constrained by the high cost of launching a satellite (i.e., getting the satellite to orbit) and the performance limitations of the launch vehicle. A satellite's available electrical power is constrained by how much power a satellite's solar array can produce.
To increase satellite payload performance capabilities, mass, and power availability should be redistributed within a satellite. Any mass or power requirement that can be eliminated from the bus portion of the satellite (i.e., the satellite payload-support portion) may be made available for the payload to use to improve its performance.
Besides the individual disadvantages of rotation sensors, star-referencing devices, horizon sensors, sun sensors and magnetometers, they also share common disadvantages. Each prior-art device requires additional power, adds weight to the satellite bus, and increases equipment costs.
Also, the consequences of equipment failure are great with prior-art devices. Without accurate attitude determination capability, a satellite is unable to perform communications functions properly. If it cannot determine its attitude, a satellite may not be able to determine where to point its communications antennas or lasers. A satellite whose attitude determination equipment has failed may have to be deorbited, potentially resulting in lost communications capability for an entire system. Additionally, satellite replacement costs (e.g., equipment and launch costs) are significant.
Thus, what is needed is an attitude determination method and apparatus that is accurate, reliable, and does not require additional expensive, power-consuming or heavy equipment to the satellite. What is further needed is an attitude determination method and apparatus that is capable of complete attitude determination at any time within a satellite's orbit without relying on other attitude-determination devices.